United States NAVSTAR Global Positioning System (GPS)

The United States NAVSTAR Global Positioning System (GPS) operates at a frequency of 1575.42MHz referred to as L1C/A. It has world-wide coverage originally using 24 satellites which began deployment in 1973 and became fully operational in 1995.

The Russian Federation Global Orbiting Navigation Satellite System (GLONASS) operates at a frequency of 1602MHz + k*562.5 (where k = -7,...,5,6) referred to as L1OF. It is a space-based system providing an alternative to US GPS. Deployment began in 1982 and was originally completed in 1995 but followed with a decline in capacity until fully restored to 24 satellites in 2011.

BeiDou Navigation Satellite System (China)

The BeiDou Navigation Satellite System covers China and is planned to begin serving global customers upon its completion in 2020.

Galileo Navigation Satellite System (EU)

Galileo is the GNSS that is currently being created by the European Union (EU) and the European GNSS Agency (GSA). The system became operational in October 2011 and as of Dec 2016 the system has 18 of 30 satellites in orbit and is expected to reach full operational capability in 2019 with the complete 30-satellite system (24 operational and 6 active spares) expected by 2020.

One Socket Protocol (OSP), aka SiRF Binary Protocol

One Socket Protocol (OSP), also known as the SiRF BINARY™ protocol, is a binary communication protocol available with the SiRFstar family of GPS receiver products from SiRF (which was purchased by CSR). In general, there are more message types and data available when using this protocol over NMEA 0183.

Note that MID 129 also specifies the period of the various NMEA messages. If changing the above data you need to also re-calculate the checksum. See ​here section 5.4 for more info on the MID 129 message

when DPGS is enabled GGA NMEA messages will show a position fix indicator of 2 to indicate a DGPS fix vs 1 to indicate a GPS fix.

use MID 170 to set SBAS parameters if you need to change the SBAS mode or region between WAAS/EGNOS/MSAS/GAGAN

W2SG0008i LNA Gain

The W2SG0008i used on most Gateworks Ventana boards defaults on power-up to use its internal high gain LNA which is appropriate for passive antennas but not necessarily appropriate for active-antennas which have their own LNA at the antenna.

While Gateworks has tested1 and found no adverse affects in terms of GPS sensitivity and accuracy when using the ​GW10044 Active GPS antenna with the default power-up high-gain internal LNA your results may vary depending on your antenna and system characteristics.

If you wish to configure the Ws2G0008i internal LNA for low-gain you must do the following according to the ​W2SG0008i datasheet section 6.4.1:

The gateworks-gps-utils.tar.gz attached at the bottom of this page contains source for a C application "sirf_osp" that will do the above in a short series of commands. After building the application for your BSP run the following commands to put the gps module into LNA low gain mode:

Antennas

GPS receivers can use active or passive GPS antennas. An active antenna is powered and uses an LNA (Low Noise Amplifier) at the antenna instead of at the GPS receiver making it more sensitive than a typical passive antenna.

Some of the GPS modules can be used with passive antenna. See the modules datasheet for more information. Gateworks provides 3.3V for the active antenna DC voltage on all boards.

Gateworks typically uses a 50 ohm MMCX connector on it's board however as a special configuration a U.FL connector can be loaded in the same location.

Here is an SMA antenna we sell in our shop that is 50 Ohm and 27-28 dBi (+- 3-4dBi) - GW10044:

Trimble Condor (Laguna product family)

Active Antenna Short Circuit Detect / Recover

All Gateworks boards with on-board GPS support active (powered) GPS antenna connections. On many boards that have a Gateworks System Controller the antenna power is periodically monitored (1Hz) for short circuit and automatically disabled/re-enabled to allow resolving the GPS antenna without power cycling the unit.

See the product manual for your board to determine if it has this capability.

Add-in cards supporting GPS

There are many GPS receivers that communicate over UART or USB-UART. Some of these are in the miniPCIe form-factor which provides USB to the connector (Note that USB is not routed to all Gateworks miniPCIe connectors - consult the product user manual for details on your board).

NMEA 0183

NMEA 0183 is a combined electrical and data specification for communication between marine electronics such as echo sounder, sonars, anemometer, gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the National Marine Electronics Association. It replaces the earlier NMEA 0180 and NMEA 0182 standards.[1] In marine applications, it is slowly being phased out in favor of the newer NMEA 2000 standard.

Most GPS receivers use this standard for communication, however typically they may only use a subset of the standard, add their own proprietary commands, and use different baudrates. Refer to the specific GPS receiver documentation being used for details.

Most GPS devices allow customizing the NMEA output such as the UART baudrate, the types of messages, and the rate at which they are sent. Please see sections above for the particular GPS device you are interested in configuring

Time synchronization via PPS (Pulse-Per-Second)

One handy feature of an on-board GPS is that it can deliver Pulse-Per-Second (PPS) signal. This signal can be used to get a high-precision time reference that an application can use to adjust system clock time.

PPS is supported by the Linux kernel by the pps-gpio driver (CONFIG_PPS_CLIENT_GPIO). This adds a pps device to /sys/class/pps which you can interact with:

# cat /sys/class/pps/pps0/assert
170026870.983207967#8

The above shows the most recent timestamp and sequence number that has been asserted via PPS

Common use is to configure the Network Time Protocol Daemon (NTPD) with a PPS source to obtain a wallclock-time with sub-millisecond synchronization to UTC.

The various Gateworks Board Support Packages includes PPS support (if a GPS is present). However, in order to hook NTPD with a PPS source, you have to have the fully featured ntpd package (i.e. not busybox). Below is an example of how to configure ntpd to use GPS and use PPS to keep syncronization:

Differential GPS (DGPS)

​Differential Global Positioning System (DGPS) is an enhancement to GPS that provides improved location accuracy from the 15-meter nominal GPS accuracy to about 10cm in the case of the best implementations.

Radio Technical Commission for Maritime Services (RTCM)

The standards applying to Differential Global Navigation Satellite Systems (DGNS) are defined by the Special Committee 104 of the ​Radio Technical Commission for Maritime Services (RTCM). Except for RTCM, there exist other proprietary DGPS standards, such as Trimble Compact Measurement Record (CMR).

Traditional land-based beacon DGPS

Traditional DGPS uses a network of fixed ground-based reference stations that broadcast the difference between their known location and the location determined via GPS. These stations would broadcast using VHF/UHF which traditionally were available on maritime vessels where GPS was originally intended to be used.

To use this you must have a receiver that can receive both GPS broadcasts as well as the VHF/UHF broadcasts from the land-based beacons. In addition you need to be in an area that has such land based beacons. The U.S. Coast Guard and Canadian Coast Guard operate these near large waterways.

Wide Area Augmentation System (WAAS)

An alternative system was developed called Wide Area Augmentation System (WAAS) which uses satellites in geostationary orbit that receive broadcasts from fixed ground-based stations. In this system the additional satellites periodically broadcast (about every 5 seconds) a Deviation Correction (DC) created from the data continually received from the ground based reference systems. This signal can be received with the same receiver used to receive signals from traditional GPS satellites.

To use this you need only a receiver that can receive GPS broadcasts, however you still need to be in an area that both has ground based reference stations and need to have at least 1 SBAS capable satellite in view. This system was created for North America however there are SBAS based systems being developed in Europe and Asia as well:

SBAS - North America / Hawaii Satellite-Based Augmentation System

EGNOS - European Geostationary Navigation Overlay Service

MSAS - Jpanase Multi-functional Satellite Augmentation System

GAGAN - Indian GPS Aided Geo Augmented Navigation

Indoor GPS Testing

Gateworks uses a GPS repeater when doing in-building GPS testing. These can be found from a variety of vendors:

An example of a test procedure carried out on our gps devices was the verification of optimal LNA mode for Ventana boards when using our standard active antenna. The procedure for this test is defined as follows: